Antigen specific suppressor factor (SF) secreted in vitro by antigen specific Ly2 plus 3 plus Ia (I-J) plus suppressor cells which can be induced in vitro with high antigen concentration abolish the function of Lyl ion helper cells (HC). This secreted SF is not genetically restricted: with antigens under immune response (Ir) gene control both responder and nonresponder mice produce specific SF, and these SF act across MHC or species barrier e.g., there is no requirement for syngeneic target cells in vitro, and human or monkey antigen specific SF can be tested on mouse HC. Serological analysis using antisera raised against antigen specific suppressor or helper factors (HF) in rabbits suggested that the functionally distinct classes of T cell factors could be distinguished by serological means. Rabbit anti-SF reacted with all antigen specific suppressor factors but not HF regardless of their antigen specificity or strain of origin (mouse, monkey or human) suggesting a "constant" region with shared or crossreacting determinants reserved during evolution. Rabbit anti-HF reacted with specific HF in a similar fashion but failed to react with SF. Mouse anti-SF only reacted with SF or HF of the same specificity as used for immunization thus recognizing determinants of the combining site. Specific SF produced by human PBL were further characterized: they act across species barrier (and were tested on mouse HC), lack Ig-determinants, bind to conA, carry DR determinants, and the serological determinants of suppressor factor invariable ("constant") part. Specific suppressor factors produced by hybrids between SC and BW tumor line were internally labelled and analyzed for the localization of the serological markers, antigen combining site, and the number of polypeptide chains in the molecule. The in vitro produced SF were shown to act in vivo, with SF derived from 2.5 x 10 to the 6th power cells (probably containing less than 0.1% of specific cells) being enough to suppress primary and secondary IgM and IgG responses in antigen specific fashion.